1. Field of the Invention
The present invention relates to a light emitting element driving circuit and, more particularly, to a light emitting element driving circuit for driving a light emitting element in a CMOS semiconductor integrated circuit used for image forming in a copy machine, laser beam printer (to be referred to as an LBP hereinafter), or the like which is required to operate with a low voltage, low driving current, and low cost.
2. Description of the Related Art
Conventionally, driving circuits for semiconductor light emitting elements have been devised to increase the pulse widths of output light signals from the semiconductor light emitting elements compared to the driving current pulse width. For example, the technique disclosed in Japanese Patent Laid-Open No. 05-152662 uses two stages of differential circuits and provides a level shift circuit between them. In addition, Japanese Patent Laid-Open No. 09-232635 discloses cascode-connected current mirror circuits. That is, a plurality of stages of current mirror circuits is provided to switch cascode connections.
FIG. 10 shows the current-to-light conversion characteristics between general driving currents and emitted light amounts in a semiconductor laser diode (to be abbreviated as LD hereinafter) as a semiconductor light emitting element. Referring to FIG. 10, a line A indicates the characteristic of the LD at fresh condition, and a line B indicates the characteristic of the LD when it has degraded due to use over time. As the LD degrades, the forward voltage of the LD which is required to obtain the same light amount increases. The forward voltage that causes the LD to start emitting light is about 1.5 to 1.6 V at fresh condition, but gradually increases to about 2.4 V as the LD degrades. In a copy machine or LBP, in order to guarantee a given number of copies, the driving circuit is required to control the desired light amount and pulse width even in a state in which the LD has degraded. In addition, with recent improvement in the performance of LDs, the light emission threshold has become several mA, which allows to obtain a desired light amount with a lower current than the past LDs.
According to the prior art described above, however, in the degraded state of the LD, since the power supply voltage required includes an operation voltage for serially connected two MOS transistors as well as a forward voltage for the LD, a 5-V system power supply is required. Therefore, a system using a 3-V power supply for a control system and a 12-V or 24-V power supply for a power system, which is a general arrangement for a recent copy machine or LBP, additionally requires a 5-V system power supply for LD driving. This leads to an increase in system cost. Furthermore, when the semiconductor integrated circuit outputs constant current pulses to the outside to drive the LD with high-speed constant current pulses, the pulses may be influenced by parasitic inductance in bonding wires, the lead frame of the package, or the like. This causes overshoot, undershoot, and ringing in the waveform of a constant current supplied to the LD, resulting in degradation of the LD. In addition, since an LD is a current-driven device, in order to obtain a predetermined light amount, it is necessary to drive the device with a constant current. As is the case with an LD forward voltage, the light emission threshold gradually increases as the LD degrades. From the viewpoint of guaranteeing a given number of copies, therefore, it is necessary to drive the LD with a constant current up to several ten mA as in the prior art. That is, it is necessary to design a MOS transistor for supplying a constant current to the LD into a size that can supply a sufficient maximum driving current. This makes it necessary to drive “the MOS transistor designed with a necessary size” with a small current in a low-current region, as described above, resulting in a degradation in the responsivity of the driving circuit. Since a parasitic capacitance including the gate capacitance of “MOS transistor designed with a necessary size” is charged, the rising time increases, resulting in a failure to obtain a desired optical pulse width. When using a cathode common LD, it is necessary to use a PMOS which is inferior in current supplying ability or current driving ability to an NMOS.